Currently OSGi tests are disabled on JDK9 and above. We should re-enable them and see what's wrong, and what can be fixed.
One problem is that we need to upgrade Karaf to 4.2.1. This PR on Hibernate Search might provide all the necessary fixes; at least it successfully make Hibernate ORM + Hibernate Search work on JDK10. See the commits starting with "HSEARCH-3151" in particular.
Another problem is that JAXB is hidden in JDK9/JDK10, and not even available in JDK11. This can be fixed temporarily with a hack similar to this one, and maybe there are more elegant solutions.
Also, it seems building the OSGi feature file under JDK11 pulls a lot more dependencies, some of which simply are not available (slf4j in particular). I didn't investigate why exactly that happens, but I suspect some global configuration which gets enabled when we build under JDK9 and above.